Since inheritance is more commonplace and more easily understood than __metaclass__, the abc module would benefit from a simple helper class:
class ABC(metaclass=ABCMeta):
pass
From a user's point-of-view, writing an abstract base call becomes simpler and clearer:
from abc import ABC, abstractmethod
class Vector(ABC):
@abstractmethod
def __iter__(self):
pass
def dot(self, other):
'Compute a dot product'
return sum(map(operator.mul, self, other))
Notice that this class seems less mysterious because it inherits from ABC rather than using __metaclass__=ABCMeta.
Also note, it has become a reasonably common practice for metaclass writers to put the __metaclass__ assignment in a class and have it get inherited rather than requiring users do the metaclass assignment themselves.

Done. I prefer to leave existing class order in documentation.
In general I agree with Eric that ABC should be before ABCMeta in the doc but now it has formalized as helper for metaclass with very short documentation.
To put ABC first we need to transplate almost all content of ABCMeta doc int ABC.
If somebody want to do it — please create new issue and feel free to make a patch.